1. Field of the Invention
This invention relates to helicopter rotor blades.
2. Description of the Prior Art
EP-A-0037633 discloses a helicopter rotor blade having enhanced retreating blade performance achieved by using aerofoil sections having different aerodynamic characteristics throughout a blade span. Thus an aft-loaded aerofoil section having a high angle of attack capability (a nose down or negative basic pitching moment coefficient) is used in an outboard region to enhance retreating blade performance, and detrimental effects of such sections are balanced by an aerofoil section with a reflex camber line (a nose up or positive basic pitching moment coefficient) in an inboard region.
The extent of the aft-loading or negative basic pitching moment coefficient (and hence the stalling angle of the rotor blade) which can be utilised in the outboard region of the prior art blade depends on the amount of reflex camber that can be used in the inboard region, and the extent of the span of the inboard region which can be devoted to balancing the moments in the outboard region. This is governed to some extent by performance limitations which dictate that the reflex camber should not extend outboard beyond about 70 percent blade radius station.
To date this has limited the negative basic pitching moment coefficient usable in the outboard region of prior art blades according to EP-A-0037633 to about -0.03.
The exemplary embodiment of the rotor blade disclosed in EP-A-0037633 describes the distributed aerofoil section invention embodied in a rotor blade constructed according to GB-A-1538055. That blade has a parallel constant chord central portion and is characterised by a rearwardly swept tip portion which in plan has a forwardly extending leading edge portion, a rearwardly swept leading edge portion, a rearwardly swept extreme tip edge and a rearwardly swept trailing edge, resulting in a chord dimension in the tip portion greater than that of the central portion. The distributed aerofoil sections of EP-A-0037633 are incorporated in inboard and outboard regions of the central portion, and this combination provides rotor blades that have proved highly successful in producing large increases in the allowable rotor operating envelope, and which were a significant factor in the establishment of the world absolute speed record for helicopters of 249.10 mph (400.81 km/hr) set by a Westland Lynx helicopter in 1986.
An objective of this invention is to further improve the performance characteristics of such helicopter rotor blades.